BATISTA JÚNIOR, Francisco Florêncio. Propriedades termodinâmicas, eletrônicas e de transporte de sistemas curvos semicondutores. 2014. 104 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-09-12T20:11:36Z No. of bitstreams: 1 2014_tese_ffbatistajunior.pdf: 17233595 bytes, checksum: 9f4c5f7e5b2bbf9bf2975c26ed164fb9 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-09-12T20:23:03Z (GMT) No. of bitstreams: 1 2014_tese_ffbatistajunior.pdf: 17233595 bytes, checksum: 9f4c5f7e5b2bbf9bf2975c26ed164fb9 (MD5) / Made available in DSpace on 2014-09-12T20:23:03Z (GMT). No. of bitstreams: 1 2014_tese_ffbatistajunior.pdf: 17233595 bytes, checksum: 9f4c5f7e5b2bbf9bf2975c26ed164fb9 (MD5) Previous issue date: 2014 / We study thermodynamic properties of an electron gas confined in a two-dimensional cylindrical surface under the action of a magnetic field perpendicular to the cylinder axis. We observed that the applied magnetic field has a similar effect to that produced by a non-homogeneous magnetic field on a flat system. We calculate the energy spectrum of the system for different values of curvature and symmetry of the magnetic field to the surface. We show that the physical properties of these systems are strongly connected to the symmetry imposed by the magnetic field by calculating the density of states, specific heat and chemical potential. We investigate how the curvature of a semiconductor film affects its electronic and transport properties. We study how the geometry-induced potential resulting exclusively from periodic ripples in the film modifies its band structure by inducing electronic confinement. For fixed curvature parameters, this confinement can be easily tuned by an external electric field, so that features of the band structure such as the energy gaps and band curvature can be controlled by an external parameter. We also show that, for some values of curvature and electric field, it is possible to obtain massless Dirac bands for a smooth structure. Moreover, we use a wave packet propagation method to demonstrate that the ripples are responsible for a significant inter-sub-band transition, specially for moderate values of the ripple height. / Neste trabalho estudamos propriedades termodinâmicas de um gás de elétrons confinado em uma superfície bidimensional cilíndrica sob a ação de um campo magnético perpendicular ao eixo do cilindro. Observamos que o campo magnético aplicado desta forma produz efeito similar ao produzido por um campo magnético não-homogêneo em um sistema plano. Calculamos o espectro de energia nesse sistema para diferentes valores de curvatura e simetria do campo com a superfície. Mostramos que as propriedades físicas desses sistemas estão fortemente ligadas a essa simetria do campo magnético através do cálculo da densidade de estados, potencial químico e calor específico do sistema. Investigamos como a curvatura de um filme semicondutor afeta suas propriedades eletrônicas e de transporte. Estudamos como o potencial efetivo induzido pelas deformações de curvatura periódicas no filme modificam sua estrutura de bandas induzindo o confinamento eletrônico. Para parâmetros fixos de curvatura, tal confinamento pode ser ajustado através de um campo elétrico externo, de modo que certas características da estrutura de bandas tais como emph{gaps} de energia e curvaturas das bandas podem ser controladas por um parâmetro externo. Também mostramos que, para alguns valores de curvatura e campo elétrico, é possível obter bandas de Dirac para filmes semicondutores com curvatura Gaussiana. Além disso, usamos um método de propagação de pacotes de onda para demonstrar que as curvaturas são responsáveis por significantes transições entre sub-bandas, especialmente para valores moderados de curvatura.

abstract: Measurements of the geometrical magnetoresistance of a conventional semiconductor, gallium arsenide (GaAs), and a more recently developed semiconductor, iron pyrite (FeS2) were measured in the Corbino disc geometry as a function of magnetic field to determine the carrier mobility (μm). These results were compared with measurements of the Hall mobility (μH) made in the Van der Pauw configuration. The scattering coefficient (ξ), defined as the ratio between magnetoresistance and Hall mobility (μm/μH), was determined experimentally for GaAs and natural pyrite from 300 K to 4.2 K. The effect of contact resistance and heating on the measurement accuracy is discussed. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2016

L'étude de cette thèse est focalisée sur la compréhension des caractéristiques du transport dans des systèmes granulaires cœur - coquille (cobalt - cobalt oxyde). Nous avons étudié le comportement magnétique et de transport de notre système granulaire ferromagnétique/antiferromagnétique. Les agrégats de cobalt de 4 nm sont fabriqués par une source à pulvérisation magnétron et condensation en phase vapeur. Lorsque les agrégats sont oxydés pendant le dépôt ils adoptent une géométrie en cœur – coquille avec un cœur icosaédrique et une coquille CoO de structure cfc. Après le dépôt, les agrégats sont caractérisés par différentes méthodes. In situ, elles sont caractérisées en fonction de la quantité déposée par le spectromètre à temps vol. Ex situ, elles sont analysées par le SQUID (mesure magnétique) et par les mesures de transport. Plusieurs caractéristiques de nos échantillons ont été mises en évidence lors de l'analyse magnétique. Trois propriétés magnétiques nous intéressent principalement. La première est le couplage d'échange F/AF, étudié sur l'ensemble des agrégats de cobalt avec différents pourcentages d'oxydation (entre 33% de CoO et 95% de CoO). Nous avons constaté que l'effet maximum est obtenu pour l'échantillon le plus oxydé à cause de l'augmentation de la coquille d'oxyde. La deuxième propriété magnétique concerne la présence d'un superparamagnétisme modifié par l'interaction d'échange entre le cœur et la coquille. La troisième est l'effet de traînage qui se manifeste le plus souvent par une diminution du décalage d'échange et du champ coercitif après plusieurs mesures successives. Ce phénomène est dû à une instabilité de la configuration magnétique obtenue lors du refroidissement sous champ mais il est cependant difficile d'identifier l'origine exacte. Les propriétés de transport de notre système sont mises en évidence par l'étude de la résistivité, de la magnétorésistance et de la résistance du Hall en fonction de la température et du champ magnétique appliqué. La variation de la résistivité avec la température présente un minimum et qu'elle varie très peu avec la température. Ce comportent est typique des systèmes transitoires, se situant entre le régime métallique et le régime isolant. La faible magnétorésistance varie très peu avec la température. Son amplitude ne dépasse pas 0.1% dans tous les alliages granulaires mesurés. Le système étudié est caractérisé par une résistivité de Hall extraordinaire positive à toutes les températures de mesure. / This study is focused on understanding the transport properties of granular systems core - shell with emphasis on cobalt - cobalt oxide materials. We studied the magnetic and transport behavior of our granular system ferromagnetic/ antiferromagnetic. The cobalt clusters of 4 nm are obtained by a magnetron sputtering source and vapor phase condensation. The clusters oxidized during deposition adopt a core – shell geometry with icosahedra core and a fcc structure of CoO shell. After deposition, the clusters are characterized by different methods. In situ, they are characterized in terms of the amount of matter deposited by time flight spectrometer. Ex situ, they are analyzed by the SQUID (magnetic measurement) and transport measurements. Several characteristics of our samples have been identified during the magnetic analyses. Three magnetic properties are mainly interest. The first is the exchange bias F/AF, studied on the cobalt clusters with different oxidation percentages (between 33%CoO and 95% of CoO). We found that the maximum effect is obtained for the the oxidized samples due to the increased oxide shell size. The second magnetic property is the presence of superparamagnetism modified by the exchange interaction between the core and the shell. The third is the training effect shown often by a decrease in the exchange bias and coercive field after several successive measurements. This might be attributed to the instability of the magnetic configuration obtained upon cooling under a field, but it is difficult to identify the exact origin. The transport properties of our system are underlined by the study of the resistivity, magnetoresistance and Hall resistance as a function of temperature and applied magnetic field. The variation of resistivity with temperature shows a minimum. In the same time this variation is small as a function of the temperature. This behavior is associated with the transition systems which are found between the metallic and insulator regime. The variation of the magnetoresistance with temperature is small. Its amplitude does not exceed 0.1% in all granular alloys measured. The studied system is characterized by a positive extraordinary Hall resistivity at all temperatures of measurement.

Neste trabalho, apresentamos os resultados de estudos com poços quânticos parabólicos (PQW, Parabolic Quantum Well ) de AlGaAs crescidos sobre substratos de GaAs pela técnica de epitaxia por feixe molecular. Medidas de transporte em PQWs do tipo n e do tipo p com larguras de 1000 ºA ate 4000 ºA em baixas temperaturas indicam um aumento abrupto do coeficiente Hall para um campo magnético critico de aproximadamente 3 T. Nosso estudo concentra-se na interpretação deste aumento observado. Com este propósito, estudamos através de cálculos autoconsistentes e de aproximações anal¶³ticas o processo de transferência de cargas em amostras com PQWs. Determinamos as densidades superficiais de cargas ns e ps, e comparamos estes resultados com os obtidos experimentalmente. Verificamos que os melhores resultados para a densidade de cargas (ns) s~ao aqueles determinados pelos cálculos autoconsistentes. No entanto, as aproximações analíticas se mostram importantes para descrever de forma qualitativa os resultados experimentais para amostras do tipo p. Numa segunda parte do nosso trabalho, estudamos a influencia da aplicação de campos magnéticos ao longo da direção de crescimento nas amostras com PQWs. Observamos uma diminuição na largura de densidade de cargas n(z) e do potencial total V (z). Estes resultados em combinação com o processo de transferência de cargas, levam a uma diminuição da densidade de portadores no poço, produto da redistribuição das cargas entre o poço e as camadas com dopagem de silencio. Desta forma, atribuímos o aumento no coeficiente Hall como sendo oriundo de uma diminuição da densidade de cargas dentro do PQW. / We present the results of experiments and calculations done on AlGaAs Parabolic Quantum Wells (PQWs) grown on GaAs by molecular beam epitaxial tecniques. Transport measurements in n-type and p-type samples with widths between 1000 ºA and 4000 ºA at low temperatures indicate an abrupt increase of the Hall coeficient at a critical field B ¼ 3 T. Our study focuses on the interpretation of this observed increase. To this end, we study by means of self-consistent numerical simulations and analytical approximations the charge transfer process in PQWs. We compare our results for the sheet densities with those observed experimentally. The best results are obtained for n-type samples for which we could numerical simulations. However, the analytical expressions we obtained also describe qualitatively the experimental results, and can be applied to p-type samples. In the second part of this work we study the efect of a magnetic feld applied perpendicular to the well. The simulations indicate a diminishing of the charge density and the total potential in the well. These results, combined with the charge transfer process, lead to a redistribution of charge between the well and the dopant layers. Therefore, we interpret the observed increase of the Hall coefcient as the result of a depletion of charge in the parabolic quantum well.

Thermoelectric devices make it possible for direct energy conversion between heat and electricity. In order to achieve a high energy conversion efficiency, materials with a high thermoelectric figure of merit (ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity) are in great demand. The standard approach is to optimize charge carrier transport while at the same time scatter the heat transport, a task that is easier said than done. Improving the electrical properties in order to increase ZT is limited since electrons also carry heat, among other reasons, resulting in higher κ with a higher σ. Low κ materials, whether through complexity or lattice distortion, are therefore of great interest in optimizing the materials’ thermoelectric properties. In this thesis I will present my investigations on certain material systems that have intrinsically low κ, materials with cage-like or layer-like crystal structure and complex chalcogenides, as well as investigations on nanostructured bulk chalcogenides in order to further lower the κ. In addition, unique transport phenomena that can be described as polaronic-type conduction and lone-pair distortion have been observed in certain materials. This too will be extensively described in this thesis.

The Shubnikov-de Haas magneto-resistance oscillations and photoconductivity were experimentally studied in order to investigate the interaction of CO laser radiation with n-InSb at liquid helium temperatures. The roles of various absorption mechanisms on these effects were considered, particularly near the intrinsic band edge. From these measurements an effective electron temperature Tₑ was defined that increased or decreased under illumination, depending upon the strength of the applied electric field.

It was the objective of this project to provide a glass cleaner formulation for a wet wipe application with cleaning and anti-fogging properties. This glass cleaner formulation was developed for automotive glass i.e. interior of windscreens. This formulation relates to a glass cleaner with a composition comprising of: (a) a blend of amphoteric surfactants; (b) a solvent system with a combination of glycol ethers; and (c) an aqueous solvent system. This glass formulation must provide good cleaning properties while also providing good wetting and sheeting properties to assist with anti-fogging properties. The objectives were obtained using 2 specific approaches: The first was by using a blend of 2 amphoteric surfactants in an alkaline medium, allowing the glass surface to become more hydrophilic which will also assist with reduction of surface tension on the glass surface. The second was by using the glycol ethers that have good coupling properties and surface tension reducing properties. The formulation was evaluated using commercial standard test methods as per the industry. A predictive model was successfully obtained for each of the five criteria that were evaluated using the 25 formulations derived from the statistical design. There were variables and variable interactions that were antagonistic for some of the criteria which were found to be synergistic for others. To achieve satisfactory cleaning, the fogging rating had to be compromised.

Intermetallic clathrates have long been of interest for materials science research. The promise these materials hold for useful applications ranges from thermoelectrics to photovoltaics and optoelectronics to potentially ultra-hard materials and magnetic cooling applications. Their unique physical properties are intimately related to their intriguing structural properties. Thus a fundamental understanding of the chemistry and physics of inorganic clathrates offers the possibility to assess their potential for use in the various applications mentioned above. The purpose of the current work is to expand the current knowledge of the synthetic routes for obtaining clathrate materials, their structural, chemical, and physical properties, particularly those that from in the type I, II and VIII crystal structures. New synthesis routes are presented and used for preparation of single crystals of Na8Si46 and Na24Si136. Single-crystal X-ray analysis, and resistivity, Seebeck coefficient and thermal conductivity measurements are presented. In addition, two "inverse" clathrates with compositions Sn24P19.3Br8 and Sn17Zn7P22Br8 have been characterized in terms of their transport properties. Since the magnetic refrigeration based on the magnetocaloric effect is a topic of great interest, type VIII Eu8Ga16Ge30 clathrates are also explored in terms of their application for magnetic cooling.

This diploma thesis is focused on fabrication of graphene field-effect transistors, characterisation of their transport properties and investigation of low-energy electron beam influence on the devices' properties under UHV conditions. The theoretical part of this work describes graphene fabrication methods, options of graphene transfer onto the substrates for graphene field-effect transistor manufacture. Furthermore, model of graphene doping via electrostatic interaction or photon, resp. electron beam exposition is explained. The experimental part of this work consist of manufacture of the graphene field-effect transistor in order to examine the change of its transport properties induced by doping of the graphene via low-energy electron beam exposition.

Thermoelectric materials can convert waste heat into electric power and thus provide a way to reduce the dependence on fossil fuels. Our aim is to model the underlying materials properties and, in particular, the transport as controlled by electrons and lattice vibrations. The goal is to develop an understanding of the thermoelectric properties of selected materials at a fundamental level. The structural, electronic, optical, and phononic properties are studied in order to tune the transport, focusing on KxRhO2, NaxRhO2, PtSb2 and Bi2Se3. The investigations are based on density functional theory as implemented in the all electron linearized augmented plane wave plus local orbitals WIEN2k and pseudo potential Quantum-ESPRESSO codes. The thermoelectric properties are derived from Boltzmann transport theory under the constant relaxation time approximation, using the BoltzTraP code. We will discuss first the changes in the electronic band structure under variation of the cation concentration in layered KxRhO2 in the 2H phase and NaxRhO2 in the 3R phase. We will also study the hydrated phase. The deformations of the RhO6 octahedra turn out to govern the thermoelectric properties, where the high Seebeck coefficient results from ”pudding mold" bands. We investigate the thermoelectric properties of electron and hole doped PtSb2, which is not a layered material but shares “pudding mold" bands. PtSb2 has a high Seebeck coefficient at room temperature, which increases significantly under As alloying by bandgap opening and reduction of the lattice thermal conductivity. Bi2Se3 (bulk and thin film) has a larger bandgap then the well-known thermoelectric material Bi2Te3, which is important at high temperature. The structural stability, electronic structure, and transport properties of one to six quintuple layers of Bi2Se3 will be discussed. We also address the effect of strain on a single quintuple layer by phonon band structures. We will analyze the electronic and transport properties of Tl-doped Bi2Se3 under strain, focusing on the giant Rashba spin splitting (Tl doping breaks the inversion symmetry in Bi2Se3) and its dependence on biaxial tensile and compressive strain.